Add ``lib/python/file_gc.py``, a minimal Odoo-shell script that calls
``ir.attachment._gc_file_store_unsafe()`` to remove filestore files
no longer referenced by any ``ir_attachment`` record.
Invoke it in ``finalize_db.sh`` after the SQL attachment purges so
that the deleted CSS/JS asset bundles, resized partner thumbnails,
and orphaned ``ir_attachment`` rows also have their backing files
cleaned from disk.
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